1. Field of the Invention
This invention relates to beta alumina electrolyte material and its manufacture and to electrochemical cells (particularly sodium sulphur cells) and other energy conversion devices employing such material.
Beta alumina electrolyte material is used as a solid electrolyte in electrochemical cells and other energy-conversion devices as a material through which sodium ions can pass. A typical example of such a cell is a sodium sulphur cell; in this case the beta alumina separates molten sodium from a cathodic reactant comprising sulphur and sodium polysulphides. During the operation of sodium sulphur cells, difficulty is sometimes encountered in wetting the beta alumina electrolyte surface with sodium. This can be overcome in some instances, by heating the beta alumina to a high temperature, for example in excess of 400.degree. C.
The present invention is concerned with improving the wetting of beta-alumina by sodium more reliably and reproducibly at lower temperatures for example 350.degree. C. which is a typical cell operating temperature.
2. Prior Art
It is known to provide capillary means on or adjacent the surface of the electrolyte material which is exposed to the sodium for drawing the liquid sodium over the surface of the solid electrolyte. In particular, it is known, for example from British Pat. No. 1511152, to provide a porous coating over the surface of the electrolyte; such a coating may be a metal coating such as a nickel or aluminium coating which may be applied for example by a plasma spray process.
Improving the wetting of the beta-alumina electrolyte provides a partial solution to the problem of the rise in electrical resistance which has often been found to occur during the operational life of a cell. This resistance rise is a change in the asymmetry of the cell resistance with time and/or cycles of charge and discharge. Only the discharge resistance increases; the charge resistance remains substantially constant. The cause of this resistance rise is not fully understood. It depends, inter alia, on the composition of the beta-alumina and appears to be primarily an interfacial effect at the interface between the electrolyte material and sodium anode.